1. Field of the Invention
The present invention relates to probes for testing electrical properties of devices. More specifically, the present invention relates to probes having field-replaceable tips.
2. Description of the Related Art
It is known in the field of high-speed, multi-channel protocol analysis to use a probe to test a device's electrical properties. The device being tested is known as a DUT (Device Under Testing). For example, a probe can be placed in parallel with the conductors transmitting the electrical signals to be tested. With this arrangement, the electrical characteristics of the DUT can be tested when the DUT is operating.
Known probe assemblies have the same basic arrangement including a DUT connected to testing equipment by a probe. The end of the probe that is to be connected to the DUT includes a connector that has a transition circuit board connected to cables. This circuit board includes a passive component, e.g., a resistor, a capacitor, etc., or a combination of passive components as needed. This use of passive components is discussed below. The cable is typically either a 50 Ω or a 75 Ω coaxial cable. The end of the probe that is to be connected to the testing equipment also includes a transition circuit board connected to the cables, where different connectors are required for different test equipment.
Specific examples of connectors used in known probes include Tyco's Mictor™ connector, Samtec's BTH and BSH connectors, and Agilent's SoffTouch™ connectors.
The probes of these known probe assemblies suffer from one or more of the following problems. A first problem is that the known probes have an effective electrical stub length that is too long. The electrical stub length of the probe is the length of the conductor in the probe through which the electrical signals from the DUT are transmitted. The physical arrangement of the conductor, especially the contact, affects the electrical signals from the DUT because of the capacitance, inductance, and resistance created by the physical arrangement of the conductor. The total disturbance of the electrical signals transmitted in the DUT caused by the electrical stub length of the probe and the physical arrangement of the conductor in the probe can be quantified as the effective electrical stub length of the probe. It is desirable to have the shortest possible effective electrical stub length to minimize the disturbance to the signal being transmitted in the DUT. In the past, it was known to shrink the physical size of the contacts, which resulted in the electrical stub length being smaller. However, the effective electrical stub length was increased because of the physical arrangement of the contact.
To decrease the electrical stub length of the probe, a method is known to provide a passive component, typically a resistor, in the path of the conductor of the probe. The passive component or combination of passive components prevents or dramatically decreases the leakage current to the probe, which allows the signals being transmitted in the DUT to be measured with minimal disturbance to these signals. However, the shortening of the electrical stub length is limited in this method by the physical arrangement of the contact. As the frequency used in devices increases, it is important to use arrangements of the probe that have smaller and smaller electrical stub lengths.
The Tyco Mictor™ connector has an effective electrical stub length of about 0.240 inches. Samtec BTH and BSH connectors have an effective electrical stub length of about 0.200 inches. The Agilent SoftTouch™ connector has an effective electrical stub length of about 0.130 inches.
A second problem is that the known probes (including the Tyco Mictor™ connector, the Samtec BTH and BSH connectors, and the Agilent SoftTouch™ connector) have a tip connector that is not field-replaceable. Because the tip is not field-replaceable, the entire probe must be sent back to the manufacturer to replace the probe tip and to calibrate the probe assembly when the tip is defective, damaged, or malfunctioning. To replace the probe tip, the probe tip must be either desoldered or cut. This is a costly and time-consuming process.
A third problem is that it is difficult to ensure the proper alignment of the known probe with the DUT. Some known probes (e.g., the Tyco Mictor™ connector and the Samtec BTH and BSH connectors) require that a connector be placed on the DUT that receives the probe. The connector increases the cost and complexity of manufacturing the DUT and increases the electrical stub length of the DUT. Some known probes (e.g., the Agilent SoftTouch™ connector) use a connector-less arrangement that includes a one-piece shroud that surrounds the entire area of the DUT with which the probe engages during the testing of the DUT. The one-piece shroud occupies a large amount of space on the surface of the DUT and must be attached to the DUT by hand.
A fourth problem with the known probes (including the Tyco Mictor™ connector, the Samtec BTH and BSH connectors, and the Agilent SoftTouch™ connector) is that they do not provide sufficient protection for the probe tip. Because the tip of the known probes does not retract, the tips are easily damaged. Typically, probe caps are provided to protect the tips. However, these caps are easily lost because they must be removed when the probe is used for testing.
A fifth problem with the known probes, e.g., including the Agilent SoftTouch™ connector, is that some of the known probes use spring pins. Spring pins include a socket, a pin that is partially disposed in the socket, and a spring disposed in the socket that pushes the pin away from the socket. This arrangement of spring pins allows the pins to travel within the socket. Because of their non-unitary construction, spring pins are difficult and time consuming to manufacture compared to contacts having a unitary construction. Because of this, spring pins are extremely expensive, for example, ten to twenty times the price of contacts having unitary construction.